These studies on rabbit lymphocytes are directed to the problems of genetic expression of cell surface markers, modulation and distribution of cell surface markers under different conditions, the relationship of modulation of cell surface components to lymphocyte activation and the relationship of histocompatibility markers to lymphocyte activation in mixed lymphocyte reactions of the rabbit. 1. The expression of immunoglobulin allotypic specificities on single rabbit lymphocytes will be examined by mixed allotype sequential stimulation and by mixed antiglobulin immunoelectronmicroscopic labelling using distinguishable markers. The question of passive uptake of immunoglobulin markers by cells will be resolved by using techniques that do not measure passively absorbed immunoglobulin (lymphocyte transformation), stripping and regeneration of surface Ig (protease, pronase) and passive transfer of thymus, lymph node or spleen cells to irradiated recipients. 2. The scanning electron microscopic localization of surface Ig and lectin receptors will be determined using immunolatex markers and the mixed antiglobulin technique. 3. The characterization of our rabbit colony for histocompatibility is now being done by mixed lymphocyte reactions. The mechanism of MLR's in the rabbit is being studied. Such experiments should provide populations of rabbits which can be used for cell transfer and tumor immunotherapy experiments.